Journal of Dairy Science Vol. 85 No. 7 1815-1828
© 2002 by American Dairy Science Association ®
Effect of Protein Level in Prepartum Diets on Metabolism and Performance of Dairy Cows1
A. F. Park*,
J. E. Shirley*,
E. C. Titgemeyer*,
M. J. Meyer*,
M. J. VanBaale* and
M. J. VandeHaar
* Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600
Department of Animal Science, Michigan State University, East Lansing 48824
Corresponding author:
J. E. Shirley; e-mail:
jshirley{at}oznet.ksu.edu.
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ABSTRACT
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Multigravid Holstein cows (n = 75) were used in a randomized block design to evaluate the effect of prepartum diets formulated to supply surplus energy and incremental concentrations of protein on the nutritional status of dairy cows at parturition. Cows were blocked according to expected calving date and assigned to one of five diets: 9.7, 11.7, 13.7, 14.7, and 16.2% crude protein (CP). Dietary treatments were initiated 28 d before expected calving date and fed until parturition. A common diet was fed postpartum. Dry matter intake and milk yield were recorded daily through 90 d postpartum. Increasing the protein concentration from 9.7 to 14.7% of dry matter during the last 28 d of gestation improved responses of cows during lactation. Increasing dietary protein up to 14.7% also increased milk yield response to recombinant bovine somatotropin (rbST) during the ninth week of lactation and yields of 305-d 2x mature equivalent milk, milk protein, and milk fat. Plasma aspartate aminotransferase tended to be highest in cows fed 13.7 and 14.7% CP prepartum, but decreased linearly postpartum in response to dietary protein levels. There were no treatment differences for plasma insulin-like growth factor-1 (IGF-1) at d 60 postpartum (before rbST provision), but IGF-1 on d 90 (after rbST provision) was higher in plasma of cows fed 14.7% CP than the other diets except 13.7% CP. Close-up diets containing 13.7% CP and surplus energy produced the most beneficial outcomes during the subsequent lactation.
Abbreviation key: AST = plasma aspartate aminotransferase, , EB = energy balance, , ECM = energy-corrected milk yield, , MUN = milk urea nitrogen, , PME = previous 305-d 2x mature equivalent milk yield, , PUN = plasma urea nitrogen
Key Words: periparturient • protein • body condition • prepartum
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INTRODUCTION
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The periparturient period (21 d prepartum to 28 d postpartum) may be the most critical time in a dairy cow’s production cycle. This period is characterized by rapid fetal growth (Eley et al., 1978; Bell et al., 1995; McNeill et al., 1997), metabolic transitions to support the ensuing lactation (Bauman and Currie, 1980), lactogenesis (Capuco et al., 1997), and ruminal adaptations to a change in diet. Improving the nutrition of the cow during this period may reduce prepartum tissue mobilization and enhance health, DMI, and milk production.
An increase in tissue reserves of the cow during the periparturient period is difficult to achieve because DMI is typically depressed during the last week before calving (Bertics et al., 1992). However, the critical issue relative to enhancing the tissue reserves at parturition is actual nutrient delivery, which is a function of DMI, nutrient density of the diet, and efficiency of nutrient utilization. Nutrient delivery becomes complicated in the transition cow because of the intake depression, which may be accompanied by a decrease in rumen volume (Forbes, 1968) and an increase in rate of passage, as observed in the beef cow (Stanley et al., 1993). The depression in DMI may not be a critical issue if nutrient delivery to the cow can be achieved by increasing the nutrient density of the diet and total digestive tract efficiency.
NRC (1989) recommendations for the dry cow do not suggest increasing the concentration of dietary nutrients as the cow approaches parturition, but it is common practice to introduce feedstuffs common to the lactation diet into the dry cow diet 14 to 21 d before parturition. This practice increases the energy and protein densities of the diet in an effort to improve the tissue reserves of the cow and acclimate the rumen microbial population as currently recommended by the NRC (2001). Grummer (1995) suggested that the energy content, particularly rumen soluble carbohydrates, of prepartum diets is more important than the protein content. Recent research (Wu et al., 1997; Putnam and Varga, 1998; Greenfield et al., 2000) has not demonstrated benefits postpartum from increasing protein in prepartum diets above NRC (1989) recommendations. All of these studies utilized energy levels above NRC (1989) recommendations; however, Putnam and Varga (1998) used energy levels considerably higher than the other two (1.72 vs. 1.5 Mcal/kg of NEL). Putnam and Varga (1998) reported an increase in nitrogen retention in late-gestation dairy cows in response to higher levels of dietary protein, but milk production was unaffected; however, prepartum intake was held constant. It is possible that postpartum benefits occur when protein is increased in prepartum diets that contain surplus energy (above 1.7 Mcal/kg of NEL) and are provided ad libitum.
The purpose of our study was to determine whether increasing dietary protein in prepartum diets high in energy would enhance the nutritional status or milk production of dairy cows.
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MATERIALS AND METHODS
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Cows and Treatments
Seventy-five multiparous Holstein cows were blocked by projected calving date and assigned to treatment groups that were balanced for BW, body condition, and previous lactation milk yield. The Cornell Net Carbohydrate and Protein System model 3.0 (Fox et al., 1992) was used to evaluate all diets on the basis of protein and to provide energy (1.54 Mcal/kg of NEL) above NRC (1989) recommendations for nonlactating, pregnant dairy cows. The experimental diets (Table 1
) were: 9.7% CP (negative control that supplied less RDP and RUP than the cow’s needs); 11.7% CP [NRC (1989) recommendations]; 13.7% CP (supplied sufficient RDP without additional RUP); 14.7% CP (same RDP as 13.7% but additional RUP); and 16.2% CP (same RDP as 13.7% but higher RUP than 14.7% CP). The experimental diets consisted of alfalfa hay, prairie hay, corn silage, common grain mix, and a top dress respective to dietary treatment. Corn grain was replaced by soybean meal to increase dietary CP. Once ruminal protein needs had been met (13.7% CP diets), expeller soybean meal (Soyplus, West Central Soy, Ralston, IA) was used to replace corn and (or) solvent soybean meal. Treatments were initiated 28 d before projected calving date and terminated at parturition. Cows not on treatment for at least 14 d before parturition were excluded from the dataset and replaced as soon as possible. Cows were fed a common diet postpartum (Table 1).
This study was conducted from late June of 1998 until early June of 1999. Cows were housed in a tie-stall facility at the Kansas State University Dairy Teaching and Research Center (Manhattan, KS) during the prepartum treatment period and the first 90 d postpartum, then moved to a free-stall facility for the remainder of their lactation. Environmental conditions in the tie-stall barn were controlled through the use of an air conditioner and heater. From dry-off until initiation of treatments, all cows were fed a common diet (DM basis) consisting of 9 kg/d of corn silage (same corn silage as used in study, harvested with a John Deere silage chopper equipped with a processor), 4.5 kg/d of a 12% CP grain mix fortified with minerals and vitamins, and free choice prairie grass hay. Cows were housed prior to treatment in either a freestall facility or on a limited brome/fescue hay pasture, depending on the season. All cows received the common diet referred to previously, regardless of housing.
Experimental diets were fed as TMR and available ad libitum by cows and allowed for 10% orts. Forages were processed in a Rota-Mix feed wagon with a particle length of 1.9 to 4.0 cm. Feed was mixed and offered once daily prepartum and twice daily during lactation. Feeding times were 0500 and 1700 h. Daily feed intakes for individual cows were recorded until d 90 of lactation. Normal herd management practices were followed during the experiment. Recombinant bST (Posilac, Monsanto, St. Louis, MO) was administered to all cows on d 63 ± 6 of lactation and every 2 wk thereafter. The Kansas State University Institutional Animal Care and Use Committee approved all practices.
Sampling and Routine Analysis
Forage samples were collected weekly and composited monthly for analysis. Grain mixes were sampled by batch and composited monthly for analysis. Samples were frozen until analyzed by Dairy One (Ithaca, NY). Nonfiber carbohydrate fraction was calculated as (100% – CP% – NDF% – fat% – ash%). Values for TDN were calculated according to Weiss et al. (1992). For concentrates, TMR, and byproducts, the NEL was calculated using the NRC (1989) dairy equations, and forage NEL was calculated using the approach of Van Soest and Fox (1992). Each respective NEL was calculated at 3x maintenance. Silage samples were analyzed weekly for DM content, and the amount fed was adjusted accordingly.
The same individual scored condition of cows weekly throughout the study (1 to 5 scale in increments of 0.25; Wildman et al., 1982). Consecutive 2-d BW measurements were obtained weekly in the a.m. during the prepartum phase and immediately after the a.m. milking during the first 90 d postpartum. Cows and calves were weighed approximately 6 h after parturition, and BW loss due to calving was calculated as the average of the most recent 2-d weights prepartum minus postcalving weight. Calving difficulty was scored on a five-point scale (1 = no problem; 2 = slight problem, 3 = needed assistance, 4 = considerable force, 5 = caesarian). Health observations were recorded daily.
Energy balance (EB) during the 28-d prepartum period (Table 2) was estimated using the equation for lactating cows (Smith et al., 1997). By substituting gravid uterus accretion energy requirements (Bell et al., 1995) for lactational energy requirements and including an estimate (VandeHaar et al., 1999) of energy required for mammary development during the last 21-d prepartum [EB = NEL intake – (0.08 * BW0.75 + 40 kcal/kg of BW0.75 + 2 Mcal)] an equation for calculating EB for dry cows was obtained.
Blood samples were taken from the coccygeal vein on d 28, 21, 10, 5, 3, and 1 before projected calving date and d 3, 7, 15, 20, 25, 60, and 90 postpartum at 0500 h. Five cows per treatment were used to determine plasma aspartate aminotransferase activity in samples obtained on d 10, 5, 3, and 1 prepartum and d 3, 7, 15, 20, 25, 60, and 90 postpartum. Blood was collected into EDTA-containing Vacutainer tubes, which were placed in an ice bath until blood was processed. Tubes were centrifuged at 500 x g for 20 min. Plasma was removed, and 3-ml aliquots were stored frozen at –20°C. Urine samples were obtained on the same days and time as blood into a 40-ml plastic container and frozen. Ketones were determined daily in fresh urine from d 10 prepartum to d 28 postpartum using Keto-stix reagent strips sensitive to acetoacetic acid (Bayer Corporation Diagnostics Division, Elkart, IN).
Udder edema was scored daily from d 28 prepartum to d 21 postpartum at the same time of day as blood and urine collections. A four-point scale was used to assess edema (1 = no edema; 2 = edema seen but not sensed through palpation, 3 = edema seen and sensed through palpation; 4 = edema present on the caudal ventral abdominal wall, udder swollen, and hard).
Cows were milked twice daily at 0530 and 1630 h. Daily milk production was measured during the first 90 d postpartum, and milk samples (a.m. and p.m. composites) were obtained weekly. Milk samples were analyzed for protein, fat, lactose, SNF, urea nitrogen (MUN), and SCC by the DHI laboratory, Manhattan, Kansas. Milk fat, protein, and lactose were determined by near-infrared spectroscopy (Bentley 2000 Infrared Milk Analyzer, Bentley Instruments, Chaska MN). MUN was determined by chemical methodology based on a modified Berthelog reaction (ChemSpec 150 Analyzer, Bentley Instruments, Chaska, MN) and SCC by flow cytometry (Somacount 500, Bentley Industries). Routine DHI monthly milk weights and composition (a.m., p.m. samples) were used to assess performance from 90 d postpartum until the end of lactation. Lactations were standardized to 305-2X-ME.
Laboratory Analyses
Plasma was analyzed for aspartate aminotransferase by a colorimetric assay described by Babson et al. (1962). Triacylglycerols were determined with a colorimetric assay (Infinity Triglycerides Reagent procedure number 343; Sigma Diagnostics, St. Louis, MO). A Technicon auto analyzer II (Technicon Industrial Systems, Tarrytown, NY) was used to measure plasma glucose, plasma urea nitrogen (PUN), total 
-amino N in plasma, and urinary creatinine. Plasma glucose was determined by a peroxidase indicator reaction with glucose oxidase (Technicon industrial method number SE-4-0036FJ4). Plasma urea nitrogen was measured by a diacetyl-monozime assay (Technicon industrial method 339-01). Total plasma -amino N was determined by a trinitrobenezenesulfonic acid assay (Technicon industrial method 512-77T). Urinary creatinine was measured by a picric acid assay (Technicon industrial method 339-11). A microcentrifugal analyzer was used for an enzymatic determination of D(–)-ß-hydroxybutyric acid and acetoacetic acid in plasma (Williamson et al., 1962). Before plasma IGF-1 analysis, IGF-1 was extracted from insulin-like growth factor binding protein by formic acid-ethanol extraction (Bruce et al., 1991). Concentration of the extracted IGF-1 was measured by radioimmunoassay using anti-human IGF-1 polyclonal antiserum (rabbit) and recombinant human IGF-1 as the standard (GroPeP, North Adelaide, Australia). Recombinant human IGF-1 was iodinated with 125I (Amersham Pharmacia Biotech Inc., Piscataway, NJ) using pre-coated IODO-GEN tubes (Pierce, Rockford, IL). Briefly, for the radioimmunoassay, standard or samples were incubated overnight at 4°C with IGF-1 polyclonal antiserum at a dilution of 1 to 150,000 and iodinated IGF-1. During the next day, killed Staphlococcus aureus protein A-positive cells (Roche, Indianapolis, IN) were added to the assay tubes (0.5 mg/tube) and incubated for 4 h at room temperature to precipitate the antibody-IGF-1 complex. Each tube was given 2 ml of assay buffer and centrifuged for 30 min at 2230 x g and 4°C. Assay buffer contained 30 mM sodium phosphate, 10 mM EDTA, 0.02% protamine sulfate, 0.02% Na azide, and 0.05% Tween-20 at pH 7.5. After centrifugation, the supernatant was decanted, and the pellets were counted in a gamma counter (TM Analytic, Inc., Tampa, FL). The intraassay and interassay coefficients of variation for the IGF-1 radioimmunoassay were 8 and 18%, respectively. Incubation blocks were nested within assay.
Statistical Analyses
The trial was a randomized block design with incomplete blocks based on expected calving date. Nineteen blocks with an average size of four cows were utilized due to a limited availability of cows with close expected calving dates. Fifteen cows were assigned to each treatment; treatments were balanced for BCS, BW, and previous 305-d 2x mature equivalent milk yield (PME). Dry matter intake and milk production data collected the day of calving were not included in the dataset because of the inherent problems associated with data collected on the day of calving. Data for statistical analyses started at 28 d before actual calving date. Analysis of variance was conducted on weekly data using SAS (1990); however, data presented in tabular form are averages based on either the late-gestation or early-lactation period. All data were analyzed as a randomized incomplete block design using the mixed procedure of SAS (Littell et al., 1996) with a split-plot analysis for variables measured over time. For variables not measured over time, the model included covariates (see below) and diet, with block being included as a random effect. For models with measures over time, the model statement included covariates (see below), diet, time, and diet * time. Block and diet * block were included as random effects. The method of Satterthwaite was used for calculation of denominator degrees of freedom for F-tests. Initial cow characteristics (BCS, BW, and PME) and interactions between initial cow characteristics and diet were included as covariates for all datasets except initial characteristics. Covariates were dropped from the model one at a time, starting with the least significant and continuing until all remaining covariates were significant (P < 0.05). Data are presented as least square means. Contrast statements for linear, quadratic, cubic, and quartic effects (based on unequal spacing of dietary protein level) were used to delineate responses to dietary protein. Prepartum and postpartum data were analyzed separately. Statistical significance was set at P < 0.05, and trends were noted at P < 0.15.
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RESULTS
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General Observations
Experimental diets (Table 1
) were formulated to contain 1.50 Mcal/kg of NEL DM (Table 3). Chemical characteristics of individual feed components and top-dress components are listed in Tables 4 and 5. Forage quality was higher than expected, thus diets averaged 1.56 Mcal/kg DM (range 1.54 to 1.59). The higher NEL content of the diets satisfied our original intent because they provided more energy than recommended by NRC (1989). The diets varied in NFC content [100 – (NDF + fat + protein + ash)] from 42% for the 9.7% CP diet to 31% for the 16.2% CP diet because soybean meal was substituted for corn to achieve incremental amounts of dietary protein (Table 1). The amount of fat increased as the amount of expeller soybean meal increased in the diet and ranged from 3.1% in the 9.7% CP diet to 3.9% in the 16.2% CP diet.
Cows used in the study are characterized in Table 6. Characteristics were similar among treatments, except those fed diets 13.7 and 14.7% CP tended to be younger than cows fed 9.7, 11.7, and 16.2% CP diets. Also, cows fed 13.7% CP had a shorter calving interval and fewer DIM during the previous lactation than those fed the other diets. The intent of this study was to initiate the experimental diets to cows 28 d before projected calving date, but variation in actual calving date resulted in treatments being fed an average of 31.7 d prepartum (Table 6).
Parturitional Events
A significant cubic effect (P < 0.05) was observed for calving difficulty score (Table 7). Cows fed 9.7 and 11.7% CP had higher calving difficulty scores than cows fed the other diets. Cows fed the low protein diets may have been limited in nutrients available to support calving. Calf birth weights were similar among treatments (mean = 41 kg) and were not considered a deciding factor in calving difficulty. Placental weights were not measured.
Health Observations
Health disorders were recorded daily and reported as the number of incidents observed (Table 2). In all cases except lameness, the reported number of incidents represents the number of cows observed with a particular disorder. In the case of lameness, a new incidence was recorded for the same cow if she appeared normal for 7 d between incidents. It is difficult to associate the incidence of lameness with the experimental diets because the affected cows experienced mild clinical laminitis during the previous lactation. Regardless, no difference was observed among treatments, and it is unlikely that lameness affected the impact of the experimental diets on performance traits.
The incidence of subclinical ketosis (Keto-stix value > 80 mg/dl) was 7, 20, 7, 0, and 27% for diets 9.7, 11.7, 13.7, 14.7, and 16.2% CP, respectively. No clinical cases of ketosis were observed. Three cows were treated for clinical milk fever (one on the 11.7% CP diet and two on the 13.7% CP diet), one cow was diagnosed with fatty liver (16.2% CP diet), none with displaced abomasal disorder, and two experienced a retained placenta (one each on diets 9.7% and 14.7% CP). Subclinical milk fever (staggered gait, droopy, cold ears) was observed in 27, 20, 0, 7, and 27% of the cows fed 9.7, 11.7, 13.7, 14.7, and 16.2% CP diets, respectively. These cows returned to normal state following one treatment with Cal-Gel (Jorgensen Laboratories, Inc., Loveland, CO). A quartic (P < 0.01) relationship was observed between diet and prepartum udder edema scores (Table 8), but postpartum udder edema scores were unaffected by dietary treatment (Table 9). Cows fed 14.7% CP exhibited less edema prepartum than other treatments.
Production Responses
A quadratic (P < 0.13) tendency best described the relationship between dietary CP and DMI prepartum (Table 8). Cows fed 14.7% CP consumed slightly more, whereas cows fed 16.2% CP consumed less than cows fed the other diets. This trend was similar, but of slightly smaller magnitude, when DMI was expressed as a percentage of BW. Cows fed 16.2% CP experienced a 29% drop in intake during the last 3 wk before parturition, whereas DMI for cows fed the other diets only decreased by an average of 20%. Prepartum treatment influenced (P < 0.05) intake and intake as a percentage of BW in the first 90 d postpartum (Table 9). Postpartum DMI was best described by a quartic response (P = 0.02) with cows fed 9.7 and 16.2% CP consuming the least DM and cows fed 11.7% CP consuming the most.
Cows gained an average of 36 kg of BW during the treatment period, with no differences among treatments (Table 8). Body condition score change was not in agreement with BW change, suggesting that frame size differed among groups. No attempt was made to measure frame size. Cows fed 14.7% CP exhibited the highest BCS at calving and had the largest gain in BCS (0.43) during the prepartum period.
Cows fed 16.2% CP were in negative EB during the last 14 d prepartum, whereas cows fed the other diets only reached negative EB during the last 7 d prepartum (Figure 1). Omitting the estimate for mammary development resulted in only cows fed the 16.2% CP in a negative EB during the last 7 d of gestation. Either our energy balance estimates were flawed or body condition scoring was not sufficiently sensitive to detect differences among treatments in EB.
Prepartum treatment influenced (cubic effect, P = 0.01) energy balance during the first 90 d postpartum (Table 9). Cows fed 14.7% CP had the lowest average postpartum EB (2.38 Mcal/d) and were in negative EB longer than cows fed the other diets (Figure 1). Prepartum treatment did not influence change in BW or BCS for the first 90 d postpartum.
Prepartum treatment influenced milk yield (Table 10) during the first 90 d (quartic, P < 0.05). Milk yield was lowest for cows fed 13.7% CP. Peak milk yield and days to peak did not differ among treatments. However, response to rbST injection during the ninth week postpartum was significantly influenced by prepartum protein level (quadratic, P < 0.05). Cows fed 13.7 and 14.7% CP exhibited the greatest response (>2 kg/d) to rbST, whereas cows fed 9.7% CP exhibited only a 0.6 kg of milk/d response during the second week after rbST injection, and cows fed 16.2% CP did not respond. The lack of response to rbST accounts for much of the difference in 305-d 2x-mature energy milk yield among treatments (quadratic response, P = 0.02). Cows fed 16.2% CP produced 1473 kg less milk during the 305-d lactation than cows fed 13.7% CP, and those fed 9.7% CP produced 1028 kg less milk than those fed 13.7% CP. Prepartum treatment had a significant quadratic affect (P < 0.01) on 305-d 2x mature equivalent milk protein yield and elicited cubic responses (P < 0.02) in 305-d 2x mature equivalent milk fat yield. Cows fed the 13.7 and 14.7% CP diets during the prepartum period had the highest yields of milk protein and fat, whereas those fed the 9.7 and 16.2% CP diets had the least. Milk protein percentage tended to be lower in milk from cows fed 9.7 and 16.2% CP during the first 90-d postpartum (quadratic, P = 0.06). Milk fat percentage was similar among treatments but, during the first 90 d of lactation, cows fed 14.7% CP produced the most fat, cows fed 13.7% CP produced the least, and cows fed the other diets were intermediate (cubic, P < 0.03). Energy-corrected milk yield (ECM) during the first 90 d postpartum was lowest for cows fed 13.7% CP and highest for cows fed 14.7% CP (cubic response, P < 0.05). Cows fed 9.7 and 14.7% CP produced the most ECM and were the most efficient, whereas those fed 11.7% CP were least efficient. Somatic cell count declined (linear, P < 0.05) as dietary CP increased from 9.7 to 14.7%, which is consistent with the recorded incidences of mastitis.
Metabolic Responses
Plasma acetoacetate was similar among diets prepartum but exhibited a cubic response (P < 0.01) postpartum (Table 11). This prepartum response of plasma acetoacetate did not agree with the quadratic response (P < 0.01) from the urinary ketones, which were high in cows fed the 9.7 and 16.2% CP diets. Cows fed 16.2% CP had the highest plasma acetoacetate concentration postpartum, which agrees with the observed incidence of subclinical ketosis. However, the plasma acetoacetate response does not agree with the urinary ketones that were high in the cows fed the 11.7 and 16.2% CP diets. Although not significantly different from cows fed the other diets, those receiving 9.7 and 16.2% CP had higher numerical values for plasma BHBA (quadratic, P = 0.18) prepartum. Unlike the plasma acetoacetate concentrations, BHBA concentrations demonstrated responses similar to urinary ketones for both the prepartum and postpartum periods. Postpartum BHBA was best described by a cubic contrast (P = 0.10), and cows fed 11.7% CP had the highest plasma BHBA concentration postpartum and the highest DMI. However, cows fed 16.2% CP had the second highest plasma BHBA concentration postpartum. Total plasma triglycerides were similar among treatments both prepartum and postpartum. Plasma glucose was not affected by treatment prepartum but was lowest in cows fed the 16.2% CP diet and highest in those fed the 14.7% CP diet postpartum (cubic, P < 0.05). Prepartum plasma total -amino N increased as dietary protein increased and was best described as a quadratic effect (P < 0.05) because plasma total -amino N plateaued at the higher levels of dietary protein. PUN increased linearly (P < 0.01) with dietary protein prepartum. Conversely, postpartum PUN concentrations were negatively related to prepartum protein (linear, P < 0.01). Plasma urea N reflects the status of the cow at one point in time, whereas MUN reflects the average PUN over the extended interval between milkings. Milk urea nitrogen (Table 10) response to treatment was best described as quadratic (P < 0.05) and did not correlate well with DMI. Prepartum urinary creatinine concentrations were not significantly different among diets but tended to be lower in cows fed 16.2% CP. The creatinine concentration in urine decreased by approximately 46, 46, 39, 37, and 16% across parturition for diets 9.7, 11.7, 13.7, 14.7, and 16.2% CP, respectively. The urine creatinine concentration among treatment groups during the postpartum period exhibited a linear response (P < 0.01) with the lowest values in urine from cows fed 11.7% CP.
Plasma aspartate amino transferase (AST) showed a tendency for a cubic (P = 0.11) response prepartum; cows fed 13.7 and 14.7% CP had the highest AST activity during this period (Figure 2). Activity of AST during the postpartum period tended to decrease linearly (P = 0.15) with dietary protein level fed during the prepartum period. Plasma AST increased an average of 38% from d 1 prepartum to d 3 postpartum. Peak AST activity occurred on d 3 postpartum from cows fed 13.7 and 14.7% CP, on d 7 postpartum in those fed 11.7 and 16.2% CP, and on d 15 in cows fed 9.7% CP.
Dietary protein level had no effect on IGF-1 prepartum. Plasma IGF-1 declined approximately 50% in all cows from d 21 prepartum through d 3 postpartum. Postpartum IGF-1 response to treatment was best described as quartic (P < 0.01). The significant treatment effect on IGF-1 during the first 90 d postpartum was primarily due to a differential response by treatment groups to rbST injection (Figure 3). No difference was observed in IGF-1 among treatment groups on d 60 postpartum (before rbST provision). However, IGF-1 was higher (P < 0.05) in plasma from cows fed 14.7% CP on d 90 (after rbST provision) compared with the other diets. Cows fed 13.7% CP exhibited a lesser IGF-1 response to rbST than those fed 14.7% CP (P < 0.05) but a response greater than those fed 9.7, 11.7, and 16.2% CP (P < 0.05).
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Figure 3. Effect of prepartum protein on plasma IGF-1 in late gestation with cows fed 9.7% CP (grid lines), 11.7% CP (dots), 13.7% CP (horizontal lines), 14.7% CP (wavy lines), and 16.2% CP (diagonal lines). Bars with unlike letters differ (P < 0.05).
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DISCUSSION
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The utilization of labile tissue protein by the dairy cow to support lactation during the early postpartum period has led some (Botts et al., 1979; Van Saun, 1991; Grummer, 1995) to theorize that improving protein reserves prepartum will reduce the incidence and (or) severity of metabolic disorders associated with parturition and, thus improve lactational performance. The most efficacious way to improve a cow’s protein status during the prepartum period is unclear. Some researchers have investigated the influence of prepartum dietary protein levels on postpartum performance (Moorby et al., 1996; Putnam and Varga, 1998; Huyler et al., 1999). Others have fed rumen-protected amino acids (Wu et al., 1997; Carson et al., 1998), varied the energy density of the diet (Flipot et al., 1988; Grum et al., 1996; Minor et al., 1998), evaluated various dietary protein-to-energy ratios (VandeHaar et al., 1999), or increased RUP (Huyler et al., 1999; Putnam and Varga, 1998; Van Saun et al., 1993) in prepartum diets. These studies have been inconclusive with regard to effects of prepartum protein on postpartum performance. Contrary to dietary influence, an injection of bST prepartum has been shown to improve protein status of dairy cows (Simmons et al., 1994).
Studies with dairy cows during the periparturient period are complicated by factors that can mask effects from the dietary treatments being evaluated in the study. We attempted to control the most obvious factors by balancing treatment groups for BCS, BW, PME, and projected calving date. Initial cow characteristics (BCS, BW, and PME) and their interactions with diet were used as covariates in the statistical model. The intent of our study was to determine whether varying the metabolizable protein supply in prepartum diets influenced body protein reserves when surplus energy [above NRC (1989) recommendations] was included in the diet. However, the source of energy shifted because the increase in dietary protein was accomplished by replacing corn grain with soybean meal that contained 4.5% fat. Diets were similar in NEL, but fat and protein replaced carbohydrate, and the diets ranged in NFC content from 42% in the lowest to 31% in the highest protein diet. Several studies (Coppock et al., 1972; Johnson and Otterby, 1981; Minor et al., 1998) have indicated that increasing dietary NFC stimulates DMI during the late prepartum period. Cows fed the 16.2% CP, 31% NFC diet consumed numerically the least DM and experienced the largest drop in intake (4.9 kg/d, 29.2%) during the last 3 wk before parturition. Because prepartum DMI and intake depression during the last week before calving was similar among cows fed the other diets, which contained higher NFC to protein ratios, the depression in intake may have been due in part to limited NFC in the diet. These results support data of VandeHaar et al. (1999) and of Minor et al. (1998) who showed an increase in DMI prepartum when NFC was increased (43.8% of DM) by adding corn grain and starch to diets containing higher CP (13.2 and 14.4%) than recommended for the dry cow (NRC, 1989).
Increasing maternal tissue reserves during the dry period has been suggested to be essential to support metabolic transitions associated with the periparturient period (Bereskin and Touchberry, 1967; Ferrell et al., 1976). Maternal weight gains of at least 0.6 to 0.7 kg/d have been recommended for the dry period to provide energy reserves necessary for lactation (Cowan et al., 1981; Broster and Broster, 1984). A gain in BW of 1.1 to 1.2 kg/d would correspond to maintenance of the cow because the gravid uterus gains about 0.7 kg/d (Bell et al., 1995) and mammary gain is around 0.4 to 0.5 kg/d (VandeHaar et al., 1999). In our study, the change in BW from wk 4 to 1 prepartum averaged 36 kg or approximately 1.7 kg/d for all cows, with no significant treatment effects. Cows appeared to gain 0.6 kg/d in BW, which is in the suggested range to support lactation requirements. The allocation of weight gain between maternal and fetal tissues appeared to be similar among treatments, as calf birth weights did not differ. Prepartum BCS tended to be influenced by prepartum treatment; with cows fed 14.7% CP experiencing the largest increase (0.43) between wk 4 and 1 prepartum.
Aldrich et al. (1993) reported that rumen microbial efficiency is maximal when the NSC (sugar and starch) to RDP ratio in lactating diets falls between 6.0 and 3.4. Our ratios of NFC (starch, sugars, pectins, and organic acids) to RDP were 7.8, 6.3, 4.8, 4.4, and 3.5 for diets containing 9.7, 11.7, 13.7, 14.7, and 16.2% CP, respectively. We purposefully utilized the 9.7 and 11.7% CP levels knowing that they were deficient in RDP to have a negative control (9.7% CP) and a treatment that met NRC (1989) recommendations (11.7% CP) to compare to diets that were sufficient in RDP (13.7, 14.7, and 16.2% CP). Increasing protein in diets sufficient in RDP was accomplished by increasing the amount of RUP in the diet. Since peak response appeared to occur at 13.7% CP, additional increases in RUP did not improve performance. This suggests that the primary consideration with respect to meeting protein requirements in prepartum diets is to meet the needs of the rumen microbial population.
The nutritional status of a dairy cow is influenced by DMI, nutrient density of the diet, and nutrient digestibility. Some (Clark and Davis, 1980; Curtis et al., 1985; Grummer, 1995) have suggested that the nutrient density of the dry cow diet during the immediate prepartum period should be increased to offset the generally observed depression in DMI. Diets for our study were formulated using the Cornell Net Carbohydrate and Protein System model 3.0 (Fox et al., 1992) and were designed to deliver sufficient ruminally available protein to support rumen function, with the exception of the 9.7 and 11.7% CP diets. The expected range in metabolizable protein was 1222 to 1628 g/d, based on an assumed DMI of 10.5 kg/d. Because one of our objectives was to determine the effect of prepartum dietary protein on DMI, intake was not regulated. The calculated metabolizable protein ranged from 1547 to 1888 g/d, based on observed intakes. Cows fed 16.2% CP consumed less and consequently had less metabolizable protein than those fed 14.7% CP. Calculated EB data (Figure 1) indicates that cows fed 16.2% CP were in negative EB during the last 2 wk before calving, whereas cows on the other diets experienced a negative EB only during the last week prepartum. Thus, cows fed the 16.2% CP diet probably mobilized more tissue reserves prepartum. This, in turn, may have reduced nutrient reserves available to support lactation and contributed to the lack of a milk response to an injection of exogenous rbST and, therefore, the lowest complete lactation milk yield. Aspartate amino transferase has been used to evaluate protein status in dairy cows (Zurek et al., 1995; Xu et al., 1998) and responds to both an increase in protein intake and tissue protein catabolism. Plasma AST activity followed a pattern similar to plasma total -amino N and protein intake in all cows except those fed the 16.2% CP diet.
Prepartum DMI for all cows was near peak on d 21 and lowest on d 1 prepartum, and PUN decreased 20, 17, 8, 7, and 2% between d 21 and d 1 prepartum for diets 9.7, 11.7, 13.7, 14.7, and 16.2% CP, respectively. The concentration of PUN is primarily a function of ruminal ammonia levels (Butler, 1998) but can be influenced by the metabolic use of amino acids for gluconeogenesis in fasting situations (Finco, 1980; Butler, 1998). The relative influence of these two factors on PUN concentration cannot be separated in our study, but observed PUN concentrations appear to reflect intake on d 1 prepartum except for cows fed the 16.2% CP diet. Their PUN remained essentially the same as it was on d 21 (14.30 and 14.03, respectively), which suggests, in the face of a decrease in DMI, that these cows were actively using amino acids to support energy needs.
A comparison of plasma BHBA concentrations between d 21 and 1 prepartum sheds some light on the influence of diet on the metabolic status of cows in our study. Plasma BHBA levels were similar among diets on d 21 but responded quadratically (P < 0.05) to protein level in the diet on d 1 prepartum (4.07, 4.06, 3.91, 4.22, and 4.00 for d 21 vs. 7.16, 5.01, 5.37, 5.78, and 6.74 for d 1 respective to diets 9.7, 11.7, 13.7, 14.7, and 16.2% CP). On d 1, cows fed diets 9.7 and 16.2% CP had the highest plasma BHBA. The increase in plasma BHBA by cows fed 9.7 and 16.2% CP between d 21 and 1 prepartum was greater than for cows fed the other diets, which agrees with the observed urinary ketones. The higher concentration of plasma BHBA in conjunction with the higher values of urinary ketones suggests that these cows were mobilizing more tissue reserves than cows fed the other diets. However, no clinical and only nine subclinical cases of ketosis were recorded for the 75 cows in this study, and four of the subclinical cases consumed the 16.2% CP diet.
Our working hypothesis was that cows adequately prepared during the dry period to withstand the demands of early lactation would have more body reserves to support sustained milk production with higher persistency than cows entering lactation with fewer nutrient reserves. As a test of this, milk and IGF-1 responses to exogenous rbST were evaluated. Cows consuming the 11.7, 13.7, and 14.7% CP treatments showed increases in milk yield in response to rbST and increases in IGF-1 concentrations between d 60 (before rbST) and 90 (after rbST). Cows fed 9.7 and 16.2% CP diets did not show significant increases in milk yield in response to the rbST injection, but all cows experienced an increase in plasma IGF-1 concentration although the increase was less for cows fed 9.7 or 16.2% CP. The greater IGF-1 concentrations of cows fed 11.7, 13.7, and 14.7% CP was assumed to be due to a greater responsiveness of IGF-1 production to rbST.
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CONCLUSIONS
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Prepartum diets were formulated to provide incremental increases in protein with the lowest levels beginning below NRC (1989) recommendations; however, all cows consumed more protein than recommended because DMI was higher than predicted. Full lactation 305-d 2x-mature equivalent milk, milk protein, and milk fat yields improved with increased prepartum protein up to the 13.7% level. Our results suggest that lactation responses of cows is improved when fed prepartum diets containing 1.54 Mcal/kg of NEL and 13.7% CP.
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ACKNOWLEDGEMENTS
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This experiment was performed in support of the objectives of the NC 185 committee. The experiment was partially funded by the Kansas Agricultural Experiment Station. The authors wish to thank West Central Soy (Ralston, IA) for donating the Soyplus and C. K. Armendariz for her laboratory assistance. The authors thank the staff at the Kansas State Dairy Teaching and Research Center for care of the cows and milk sample collections.
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FOOTNOTES
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1 Contribution Number 02-14-J, Kansas Agriculture Experiment Station, Manhattan, KS 66506. 
Received for publication November 15, 2001.
Accepted for publication January 29, 2002.
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TOP
ABSTRACT
INTRODUCTION
), 11.7% CP (
), 13.7% CP (
), 14.7% CP (•), and 16.2% CP (
). Effects in model: linear contrast (P < 0.07) prepartum and cubic contrast (P < 0.01) postpartum. Energy balance prepartum equals NEL intake minus fetal energy (FE) minus net energy for maintenance (NEM). NEM calculated as 0.08 x BW0.75. FE calculated as 40 kcal/kg of BW0.75 (